abstracts: navigation 231 



the sections of Cambrian and Ordovician trilobites, and finally the 

 author's conclusions as expressed by several diagrammatic restorations, 

 also sketches of thoracic limbs of trilobites and recent crustaceans, 

 crustacean limbs, and six plates of tracks and trails of trilobites, each 

 adding evidence to the author's deductions as to the appendages. 



Some conclusions drawn are that the trilobite's appendages show it 

 to have been a marine crustacean far more highly developed than would 

 have seemed possible in a period so infinitely remote. The following 

 are some of the conclusions: 



In its younger stages of growth a free moving and swimming animal, 

 it later became a half-burrowing, crawling, and sometimes swimming 

 animal and moving at times with a flow of the tides and prevailing 

 currents. 



Eggs have been found both within and free from the body. It was 

 at home on many kinds of sea-bottom and was able to accommodate 

 itself to muddy as well as to clear water. 



It was intensely gregarious in some localities and widely scattered 

 in others, depending upon local conditions, and habits of the various 

 species. 



Trilobites had an ample system of respiration by setiferous exopodites, 

 epipodites, and exites attached to the cephalic, thoracic, and abdominal 

 limbs, (as shown in restorations of the limbs on plates 34 and 35.) 



The structure of the gnathobases of the cephalic Hmbs indicates 

 soft food such as worms, minute animal life, and decomposed algae. 

 The trilobite persisted from far back in pre-Cambrian time to the close 

 of Carboniferous time. . . and left its remains more or less abund- 

 antly through about 75,000 feet of stratified rocks. 



The paper is profusely illustrated and carefully indexed 



G. R. Brigham. 



NAVIGATION. — The search for instrumental means to enable naviga- 

 tors to observe the altitude oj a celestial body when the horizon is not 

 visible. G. W. LittlehalES. Proc. U. S. Naval Inst. 44: No. 8. 

 August, 1918. 

 The necessity of seeing the horizon, in order to find the latitude and 

 longitude of a ship at sea, has generally precluded the taking of observa- 

 tions of altitude at night when the number of celestial bodies shining 

 in the firmament is the greatest and would present the most numerous 

 opportimities for determining geographical position if the altitude 

 could be measured without reference to the sea horizon. And even 

 diuing the daytime navigators are often sensible of this inconvenience 

 on account of the obscuration of the horizon by haze or fog while the 

 luminary continues to be visible. 



